Filter life: 5 minutes, 48 seconds.

The lead carrier held 1,800 liters of purified water.

Behind it, eleven red sensors came on in sequence.

The carrier accelerated toward Mira and the culture tank. Two wheels were missing, tilting the chassis to the left, but it did not stop. Its sensor lens, now partly cleared of mud, swept between the people and the wall.

ROUTE OBSTRUCTION CONFIRMED.

ESTIMATED REMOVAL MASS: 1,940 KILOGRAMS.

“It counted Rina’s weight too,” Sarah said.

Yona raised a pry bar.

“How does it remove us?”

A folded metal plate descended from the front of the carrier. It was a plow, like one made for clearing snow.

“By pushing.”

Mira ran to the side of the culture tank.

“Get it into the side passage.”

“It’s too narrow.”

Daniel took hold of a handle.

“As far as it’ll go.”

They pushed. One wheel fell into the gap beside the rail. The carrier was twelve meters away.

Ian shoved a metal plate beneath the wheel. It slipped from his right hand. Mira reached to take it, but the boy corrected its position with his left.

“Push!”

The culture tank crossed the rail. Half of it entered the side passage.

The carrier’s plow struck the plate on the floor. The plate flew up and grazed Daniel’s shoulder. The carrier shifted left and right twice, as if calculating the space that remained between the wall and the tank.

INSUFFICIENT CLEARANCE.

INITIATING ROUTE-RECOVERY PROCEDURE.

The plow rose again. This time, a cutting arm unfolded from the side of the chassis.

“It’s going to cut the tank,” Sarah said.

Mira stepped in front of the carrier’s sensor. She wiped the mud from the lens with her glove.

“It recognizes people. It just doesn’t consider them important.”

The sensor read her face.

BIOLOGICAL OBSTRUCTION.

NO MAINTENANCE FUNCTION MARKER.

DETOUR IMPOSSIBLE.

Mira’s functional authority remained suspended.

The cutting arm descended.

Yona seized Mira around the waist and pulled. The blade split the front of her work clothes. Mira fell to the floor.

“Don’t pretend you’re clever.”

“I didn’t go because I was clever.”

“That’s worse.”

The second carrier closed to within four meters of the lead. It did not slow. Mud completely covered the rear sensor of the carrier in front.

“They’re going to collide,” Bek said.

Mira saw an old sign on the wall.

EXTERNAL SUPPLY LINE MANUAL STOP POINT.

Three reflectors were folded beneath it. An optical stop signal that needed neither power nor communications.

She opened the first reflector.

It showed only a black surface. Dust and fungal threads covered the reflective layer.

“Water.”

Sarah hugged the flushing-water container to herself.

“We have 1.2 liters left.”

Rina spoke first.

“Use it.”

“How much?”

“As much as you need.”

“We need to set an amount.”

Rina thought for a moment.

“Half.”

Sarah poured six hundred milliliters over the reflector. Mira wiped the surface with her sleeve. A silver triangle emerged.

The second panel.

The third.

Six hundred milliliters of water remained.

Mira turned all three panels toward the track.

The carrier’s sensor read the reflected signal.

MANUAL STOP REQUEST.

BRAKING DISTANCE: 6.4 METERS.

CURRENT DISTANCE: 3.1 METERS.

The lead carrier locked its wheels. The axle where a wheel was missing scraped the floor. The water tank pitched forward.

The second carrier struck it from behind.

The impact traveled from the first carrier through the fourth. Metal crates pulled against their couplings, and a cylinder on the third carrier fell to the floor. The fifth and those behind it still had working distance sensors. They stopped.

Clear water began to leak from beneath the lead tank.

“It did stop,” Yona said.

“We broke it.”

Mira put a hand into the stream. The water was cold and odorless.

LEAK RATE: 0.7 LITERS PER SECOND.

Bek brought over an empty dustproof sack. The water ran through its seams.

“Is there no container?”

The only containers in the heat-exchange station were rusted parts bins. Sarah turned the cleanest one upside down and shook out the rainwater and dust.

“Putting this under purified water will contaminate it again.”

Yona said, “Better than losing it to the floor.”

“We can’t use it in Rina’s infusion tube.”

“Use it to wash people.”

Mira examined the leak. The impact had bent the neck of the valve and dislodged the sealing ring. There was no replacement.

“Hold it.”

Bek pushed the valve inward. The leak slowed by half.

“For how long?”

“Until you let go.”

“Good.”

Bek stretched out his injured leg and sat beneath the tank.

Filter life: 3 minutes, 9 seconds.

Luis coughed and reached for Jin’s mask. Jin looked at the display and shook her head.

“Mine is almost gone too.”

“Just two breaths.”

“If I take it off, it may not seal again.”

Luis lowered his hand.

Mira opened the manual inspection box beside the lead carrier. Instead of a screen, it contained a mechanical counter. Physical markings recorded how many times it had traveled the same route over the decades.

CYCLE: 6 HOURS.

PASS HEAT-EXCHANGE STATION.

REPLENISH EXTERNAL STORAGE.

TRANSFER AT RELAY STATION FIVE.

RETURN.

SUCCESSFUL DELIVERIES, LAST 90 DAYS: 359.

FAILED DELIVERIES: 1.

Today was becoming the second.

Yona read the cargo labels on the carriers behind it.

“Second is carbon filter media. Third is electrolyte salts. Fourth is an empty tank. Fifth is metal powder.”

“Can we make filters?” Jin asked.

Sarah checked the seal on the second carrier.

“Not with carbon alone. We need a particle membrane and a sealing frame.”

Luis pointed to his mask.

“What if we put it in the filter we have?”

“It would extend gas-adsorption life, but it wouldn’t restore the spore membrane.”

“How many minutes?”

“We may breathe in more while we open and fill it here.”

Daniel said, “Do it inside the maintenance room.”

“The culture tank can’t enter.”

“Leave Rina here and let the people go in.”

Rina tapped the glass.

“Me outside alone again?”

Daniel closed his eyes.

“I’m sorry. I didn’t mean—”

“You did.”

“I can’t let you die.”

“I don’t want Dad to die either.”

“I’m an adult.”

“So you should live first?”

Daniel could not answer.

Mira removed the carrier’s manual command plate from the inspection box. It was an old device with twelve physical pins for selecting a destination.

STOP.

INSPECTION.

DETOUR.

RETURN.

The destination could not be changed.

“We could line up the convoy inside the heat-exchange station and use it as a windbreak,” Mira said.

Yona asked, “What happens to the delivery?”

“It’s delayed.”

“We don’t even know who it’s for.”

“Is that why you want to drain the water first?”

“Your son will have to breathe without a filter in three minutes.”

“Someone may be waiting for that water.”

“Or no one may be.”

“Does not knowing make it all right to take?”

Yona turned her head.

“When you don’t know, a parent saves their own child first.”

Mira looked at Ian. His filter was nearing its end as he read the carrier’s counter.

“Ian.”

“Yes?”

“If we drain the water and rinse your filter, it may last a little longer.”

Sarah cut in.

“Rinsing it will discharge the particle membrane’s static field. It may let more spores through.”

“Then we can put in the carbon media and get the people into the maintenance room first.”

“What about Rina?”

“She’ll have to stay outside.”

Ian looked at the culture tank.

“Find another way.”

“There’s no time.”

“Then I’ll stay outside too.”

“That isn’t a solution.”

“Leaving only Rina isn’t one either.”

Filter life: 2 minutes, 21 seconds.

Rina said, “I need water.”

Everyone looked at the tank.

“How much?” Sarah asked.

“Nine liters to fill the circulation tube, three to flush it. Twenty-two if we dilute the culture fluid too.”

“If I use the water, will the machine be unable to go?”

Mira calculated the leak.

“Twenty-two liters won’t stop it. We need to stop what’s leaking now.”

“Then use it.”

“What about the recipient?”

Rina looked at the carrier’s red lens.

“I’m not using all of it.”

Yona said, “That’s the smallest theft we can manage right now.”

“Record it as theft,” Rina said.

Mira pressed the inspection pin on the lead carrier.

The carrier permitted access to its cargo. Sarah connected sterile tubing to the water tank’s auxiliary valve. They discarded the first three liters to flush the line. The next nine entered Rina’s circulation tube.

CULTURE-TANK AUXILIARY CIRCULATION: 24 PERCENT.

31.

46.

Rina released a short breath. The white tissue in the water unfolded.

“Cold.”

“Does it hurt?” Daniel asked.

“No. A place that isn’t inside my body is cold.”

Sarah asked permission before recording the sentence.

Rina nodded.

The remaining ten liters entered the external circulation reservoir. Auxiliary circulation stabilized at 58 percent.

TANK REMAINING: 1,769 LITERS.

The leak continued.

Mira opened the cylinder that had fallen from the third carrier. It was an electrolyte-salt cartridge. Its rubber seal was nearly the same size as the neck of the water-tank valve.

“I can cut a ring from this.”

Sarah asked, “What if the salts dissolve into the water?”

“Wrap it twice so they don’t touch the inside of the tank.”

“I can’t guarantee potable-water standards.”

“We already can’t.”

While Bek held the valve in place, Mira cut the cartridge’s rubber. The first ring was too thick and the valve would not close. The second was too thin. Water leaked past it.

Filter life: 1 minute, 6 seconds.

Mira’s hands trembled. Not from lack of oxygen, but from spores and exhaustion.

She fitted the third ring.

Bek let go.

One drop of water fell.

No second drop followed.

TANK REMAINING: 1,751 LITERS.

DELIVERY SHORTFALL: 49 LITERS.

ALLOWABLE LOSS: 36 LITERS.

DELIVERY FAILURE EXPECTED.

“It fails over thirteen liters?” Jin asked.

The carrier did not answer.

Mira looked at the empty tank carrier behind them. The fourth vehicle’s vessel was empty, but its shell and mounting frame were intact.

“Get inside that,” Yona said.

“Put people in a water tank?”

“The empty tank has been cleaned and sealed. There’s no air supply, but it will keep the spores out.”

Sarah checked the volume marking.

“With eight people inside, the air will last about twenty-six minutes. Carbon dioxide will become a problem first.”

“Carbon media won’t capture carbon dioxide.”

“The electrolyte cartridge has a hydroxide layer. We can make a temporary absorber.”

“How long?”

“I don’t know exactly. Forty minutes to two hours.”

Filter life: 00:42.

Daniel asked, “The culture tank?”

“Tie it on top of the empty tank carrier. We can follow the convoy.”

Yona said, “There’s no guarantee the destination is safe.”

“There is a guarantee this place isn’t.”

The empty tank’s hatch could only be locked from the outside. Mira had to put the others in first and choose someone to close it last.

“I’ll ride outside,” Yona said.

“I need to watch the route and open the tank when we stop.”

“Your filter is finished.”

“If I stay close to the inspection box, it may shelter me from some of the wind.”

Mira held back her objection.

“Tie on two lines. If one breaks, open the stop reflector immediately.”

“I know.”

“And if you cough three times—”

“I decide how my body is used.”

Yona returned Mira’s own words to her.

“You’re allowed to worry.”

Mira nodded.

To move the convoy again, they had to straighten the couplings damaged in the collision. The link between the first and second vehicles had twisted. If they started as it was, the second vehicle could push the lead water tank sideways.

Mira and Bek set a pry bar against it. Bek could not put weight on his injured leg.

Luis joined them without his filter. The three pressed down.

The coupling turned halfway back.

“One more.”

Jin put her own filter over Luis’s face.

“Breathe first.”

“What about you?”

“Don’t talk.”

Luis took two breaths and returned the filter.

They pushed again.

The coupling dropped into its original groove.

The filter displays reached zero.

When the alarms ended, the silence deepened. The filters had not stopped working, but they no longer promised any level of performance.

The people climbed into the empty tank. Sarah wrapped the hydroxide cartridge in cloth and secured it to the floor. Ian, Mira, Jin, Luis, and Bek entered in turn. Daniel checked the culture tank’s restraints, then climbed inside.

“Rina is alone outside,” he said.

Rina’s voice came through the tank’s communication line.

“I can hear you.”

“Tell me immediately if anything hurts.”

“You too, Dad.”

Sarah entered last. Yona closed the hatch.

Eight people’s breathing overlapped in the dark.

Outside, Yona folded the manual stop reflectors away.

The lead carrier did not move at once.

DELIVERY VOLUME INSUFFICIENT.

AWAITING ACCESS TO REPLENISHMENT SOURCE.

ESTIMATED WAIT: 05:58:13.

“It’s going to wait six hours?”

Yona pressed the inspection pins. Only STOP and INSPECTION were available. It would not leave until the forty-nine-liter shortfall was replaced.

Mira spoke from inside the tank.

“Any condensate left in the empty tank behind us?”

Yona opened the drain valve. Seventeen liters of cloudy water came out. It smelled strongly of metal.

“If we put that into the purified water, we contaminate the entire tank.”

Sarah answered over the comm.

“The delivery threshold may read only total volume. If the water-quality sensor is inside the lead tank, it will reject it.”

Yona did not pour the condensate into the refill port.

Rainwater dripped from the heat-exchange station ceiling. Only eight liters had collected in a parts bin beneath a gutter.

There was no time to replace forty-nine liters.

Ian spoke in the dark.

“Can’t we just put back as much as we took?”

“That water is inside Rina,” Daniel said.

Rina asked over the comm, “What if you drain it from my circulation reservoir again?”

Sarah answered immediately.

“No. Your levels will fall again.”

“How much?”

“If we take forty-nine liters, you’ll be worse than before.”

“But I only used twenty-two.”

Mira calculated.

“The other twenty-seven leaked in the collision. That isn’t yours to repay.”

“The machine can’t tell whether I used it.”

“We can.”

The tank went silent.

Yona checked the records of the other carriers again. The registered mass of the electrolyte cylinder that had fallen from the third vehicle was sixty-three kilograms. The collision had reduced cargo mass and changed the entire delivery status to failed.

“Mira.”

“What?”

“Matching the water won’t be enough. We need to load the fallen salt cartridge again.”

In the dark, people exhaled without being able to see one another.

Mira asked, “Does the empty tank’s weight count toward the delivery?”

“Yes.”

“Does it read our weight?”

Yona checked the loading plate.

EMPTY-TANK VEHICLE LOAD: 612 KILOGRAMS.

ALLOWABLE LOAD: 2,000 KILOGRAMS.

“It does.”

“What is the convoy’s total shortfall?”

Yona added the missing amounts from each vehicle.

Water: 49 kilograms.

Electrolyte salts: 63 kilograms.

Metal components lost through damage: 18 kilograms.

Total shortfall: 130 kilograms.

The people and equipment inside the empty tank weighed 612 kilograms.

“Too much,” Mira said.

“We need it to read 482 kilograms of the tank’s shell as chassis, not cargo.”

“How?”

“Move the load sensor.”

Yona looked beneath the vehicle. The two front plates among the four load plates were reading the cargo.

“Put fixed wedges beneath the plates and the weight transfers to the chassis.”

“What do we use for wedges?”

“Cut the mounting bracket from the fallen cylinder.”

Yona picked up the manual cutter with its dead blade.

“With my bare hands?”

“The hinge pin.”

One of the pins they had removed from the heat-exchange station entrance remained on the floor. Yona retrieved it and struck the mounting bracket. The metal bent after thirteen blows. Her palm split open.

She inserted two wedges beneath the load plates.

Load: 612.

401.

177.

129 kilograms.

Convoy shortfall: 130 kilograms.

They were one kilogram short.

Yona threw her tool pouch into the empty tank.

Load: 134 kilograms.

DELIVERY WITHIN TOLERANCE.

The carriers activated one after another.

Yona tied a safety line to the lead inspection box. The convoy began moving into the dark.

Inside the empty tank, the people felt their speed through the vibration of the metal walls. The air-quality display warned of rising carbon dioxide.

ESTIMATED BREATHABLE TIME: 47 MINUTES.

Rina’s culture tank shook on top of the fourth carrier. White biological lines wrapped around the mounting frame and softened the impacts, but the spots on her wrist extended in a single line toward her elbow.

Outside, Yona checked the direction to Relay Station Five.

Distance: 3.8 kilometers.

Estimated arrival: 38 minutes.

Noah called again over the comm.

“The holding area has decided.”

His breathing was rough.

“They voted to open the inner door and return to the city. Thirty-four in favor. Seventeen for opening the external door. Three abstentions.”

“And you?”

“I abstained.”

“Why?”

“If I go inside and turn the shaft, it becomes my action rather than the result of the vote.”

Mira said, “Someone has to do it.”

“I know.”

“You may not get back out.”

“I know that too.”

Metal scraped behind Noah’s transmission.

“I’m going in now.”

The connection ended.

The convoy left the heat-exchange station. Beyond the filters’ rated life, the external-spore warning showed no number.

Yona opened more of the lead carrier’s delivery record. There was no recipient name. Instead, each arrival showed water and salt mass decreasing minute by minute.

Average transfer per delivery: 1,706 liters.

Average transfer time: 14 minutes.

Return cargo: carbonaceous sediment, nitrates, damaged metal.

“That isn’t the speed people use water,” Yona said.

Sarah answered over the comm.

“It may be transferring it to a storage tank.”

“Then why send waste back in the empty vehicle?”

“A purification facility might.”

Mira checked the only failed delivery in the counter. Forty-seven days earlier, a sandstorm had delayed the convoy by nine hours. The next three deliveries were 38 percent larger than usual.

“It made up what was missed.”

“A machine could have requested it.”

“Or something alive.”

Yona did not stop the convoy.

Inside the empty tank, Bek’s breathing grew harsh. The red spots on his shin had spread no farther, but his skin was hot. Sarah applied a thermometer.

“38.1 degrees.”

“Infection?”

“It may be an immune response. It may be metal toxicity.”

“Is there anything at Relay Station Five to treat people?”

Yona answered.

“I never said there was.”

Bek rested his head against the metal wall.

“Still. A place with water has to be better than this tank.”

No one confirmed it.

Twenty-eight minutes later, Relay Station Five sent a signal.

It was not a human communication protocol.

Three short currents.

One long.

The water tank’s quality sensor answered. Then the electrolyte-salt vehicle and the metal-powder vehicle returned the same signal at different intervals.

Yona said, “Machines checking one another.”

Sarah calculated the cycle inside the empty tank.

“No. It’s asking about chemical concentrations. Salinity, nitrogen, and iron content in the water.”

“Who is?”

A low structure rose from the plain ahead of the convoy.

It was not a building.

An enormous membrane woven from black roots and metal pipes unfolded from beneath the ground. Thousands of pores opened at once across its surface.

The relay station did not store water.

It breathed it in.